Vaporator refrigerant feed modulated from a variable load

ABSTRACT

A gravity circulation flooded evaporator has a first control which is responsive to the pressure of the liquid head in the supply line to the evaporator and a second control in the suction line to the compressor which is responsive to the vapor flow rate, the two controls being interconnected in such manner that the second is operative to override the first in order to modulate or stop the flow of refrigerant to the evaporator at a lower pressure head than that which is normally used during periods when the load on the evaporator is below a predetermined minimum, in order to avoid an undesirable accumulation of liquid refrigerant in the evaporator during periods of low load.

United States Patent Garland 1 1 Aug. 1,1972

154] VAPORATOR REFRIGERANT FEED MODULATED FROM A VARIABLE LOAD Inventor: Milton W. Garland, Waynesboro,

Assignee: Frick Company, Waynesboro, Pa. Filed: Dec. 7, 1970 Appl. No.: 95,594

References Cited UNITED STATES PATENTS 1,880,653 10/1932 Baars ..62/2l9X 2,114,128 4/1938 Smith ..62/DIG.21

2,163,799 6/1939 Nevin ..62/2l7 2,191,623 2/1940 Philipp ..62/DlG.2l 3,315,481 4/1967 Checketal ..62/ll5 Primary ExaminerWil1iam F. ODea Assistant Examiner-P. D. Ferguson Attorney-A. Yates Dowell and A. Yates Dowell, Jr.

[5 7] ABSTRACT A gravity circulation flooded evaporator has a first control which is responsive to the pressure of the liquid head in the supply line to the evaporator and a second control in the suction line to the compressor which is responsive to the vapor flow rate, the two controls being interconnected in such manner that the second is operative to override the first in order to modulate or stop the flow of refrigerant to the evaporator at a lower pressure head than that which is normally used during periods when the load on the evaporator is below a predetermined minimum, in order to avoid an undesirable accumulation of liquid refrigerant in the evaporator during periods of low load.

6 Claims, 1 Drawing Figure I EVAPORATO/Q /d J I /f l Ira/mam! VAPORATOR REFRIGERANT FEED MODULATED FROM A VARIABLE LOAD BACKGROUND OF THE INVENTION tive of such a system. A purpose in such a system is to provide a nonboilirig liquid refrigerant head in the down supply line to the evaporator which (by gravity) cess refrigerant may enter an evaporator during periods causes the boiling vapors in the evaporator to move in the desired direction only.

An evaporator has a large percentage of vapor relative to liquid refrigerant under its ordinary load conditions. For example, for each ton of refrigerant at F, with ammonia refrigerant, there is evaporated 0.434 pounds of liquid ammonia (0.0135 cubic feet by volume)-which produces 3.96 cubicfeet of vapor, or in other words, a ratio of one volume of liquid for 293 volumes ofvapor.

In designing the layout of such evaporators, the height of both the supply or accumulator tank and the float control must be such as to create the necessary flow rates at maximum refrigeration load. However, during periods of low loads or of no loads, the evaporator becomes filled with a higher ratio of liquid than vapor. When a substantial load is again applied, as by restarting a fan of an air unit or circulating liquid in a chiller, a large vapor volume is suddenly generated which forces a volume of liquid into an accumulator which may be beyond the separation capacity of the accumulator, thereby permitting liquid refrigerant to reach and possibly damage the compressor.

A system which is applicable to a single evaporator only is described in the patent to Check et al U.S. Pat. No. 3,315,481. This system serves to maintain a liquid seal in the receiver and to block refrigerant flow during starting in order to obtain a pressure build up for control cooling. Also, under low load conditions, high pressure vapors from the condenser are bypassed into the evaporator and introduced under the liquid so as to ob-' tain a greater liquid boiling rate in order to wet the tubes. However, this patent describes a single evaporator fixed charge system in which over-flooding at low loads does not present a hazardous condition when full loads are reapplied.

The patent to Johnson et al, U.S. Pat. No. 3,400,552, describes a system for controlling the amount of refrigerant in a single evaporator system. The purpose is to sense an oversupply to the evaporator following a low load and then a sudden load of substantial size, attempting to correct by bleeding refrigerant from the system.

SUMMARY OF THE INVENTION AND OBJECTS and therefore, unless proper controls are provided, ex-

of low load or no load periods and can cause floodback into the compressor when operation under higher load is commenced.

Accordingly, it is an object of the invention to provide controls for refrigeration systems which are responsive not only to the demand for feeding of refrigerant to the evaporator, but also sense the flow rate from the evaporator back to the compressor and which reduces the feed in response to a low flow rate in order to avoid overfilling of the evaporator during a low load period. I

The control is applicable to individual evaporators in any single or multiple evaporator system and functions independently of the system pressure differential.

' In the illustrated embodiment the. objects of the invention are accomplished by a first differential pressure liquid head cell which is responsive to the head of parent from the following description considered in conjunction with the accompanying drawing in which:

The FIGURE is a diagrammatic representation of a system employing an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODlMENT With further reference to the drawing, a refrigeration system is disclosed including a compressor 10 having a discharge line 11 and suction line 12. Line 11 is connected to condenser 13, line 14 and receiver 15 to supply line 16. Line 16 is connected to feed valve 18 which supplies accumulator 19 through line 20.

. Accumulator 19 has a supply, or down leg, 21 which is connected to an evaporator 22 of the flooded type having a return line 23 to the upper portion of the accumulator. I

A differential pressure valve 25 with bellows 26 is connected by line 27 to line 21 adjacent to the bottom of the accumulator and its bellows 28 is connected by line 29 to line 21 at a level preferably below that of the bottom of the evaporator. The bellows 26 and 28 act on opposite sides of a movable member 30 having a metering tip 31 which is received in an orifice 32 of the valve housing. Bellows 28 is assisted by spring 33 tending to urge the movable member toward closed position. Orifice 32 is connected to an air inlet line 35 from a supply line 36 whose nominal pressure is ordinarily approximately 25-30 p.s.i.g.

During normal operation, when there is a full load and a full head of liquid refrigerant in the line 21, the pressure head on line 29 will tend to close valve 25,. In its open position the valve permits a variable passage of air from the line 35 through the valve into line 38 and thence into branch line 39 into feed valve 18 to act downwardly on its diaphragm 40. The diaphragm is connected to valve member 41 which is normally urged closed by spring 42. However, when the air pressure from line 39 is sufficient the valve is open to permit the flow of refrigerant from line 16 through the valve and from line 20 to the accumulator. When valve 25 is partially or entirely closed, however, the air pressure on the diaphragm 40 is decreased, thereby tending to close the valve member 41 and shut off the flow of refrigerant to the accumulator.

The foregoing is representative of a refrigerant level control as has been used heretofore.

During operation at ordinary loads, the height of the accumulator above the evaporator produces a sufficient liquid head on the evaporator to maintain the necessary amount of refrigerant therein for flooded operation and also to insure that the boiling vapor is discharged through the line 23 instead of backing up into the line 21.

Since the compressor may supply additional accumulator.

In the event the load on the evaporator decreases substantially below a predetermined minimum, the

evaporators as, for example, evaporator 22 through branch line 16', the amount of refrigerant available for circulation to any given evaporator isvariable and may under conditions of low load result in the accumulation of an excess amount of liquid refrigerant in an evaporator unless means is provided to avoid this result. Accordingly, the control means which will be described provides for the necessary supply and pressure head of liquid refrigerant to the evaporator duringv normal operation, but reduces the supply when the load on the evaporator is below a predetermined minimum in order to avoid the accumulation of an excess of liquid refrigerant in the evaporator.

The system illustrated includes a differential pressure control valve 50 having a first inlet line 51 and a second inlet line 52 which are on opposite sides of a restrictor 53 in the suction line 12 from the evaporator. The lines 51 and 52 are connected to bellows 54, 55, respectively, which engage a movable valve member 56in order to position a metering tip 57 which is positioned in a metering orifice 58 in the valve housing connected to air line 36. An outlet line 61 is also connected to the housing. Valve member 56 is urged toward closed position by spring 59 which assists bellows 55. Line 38 from valve 25 is connected through a bleed relay valve 60 to the line 61.

Valve 60 has a bellows 62 which is connected to a metering tip 63 engaging orifice 64 which is connected to line 38 from valve 25. Spring 65 mounted on the shaft of the tip 63 tends to open the valve against the pressure of the bellows 62. Orifice 66 in the valve housing has a metering screw 67 which may be adjusted to permit bleeding of the valve 60 at the desired pressure.

The tension of the spring 65 may be adjusted by nut 68 on the shaft in order to insure that the air pressure to the feed valve 18 from valve 25 does not drop below a predetermined minimum. The purpose of this is to provide that in the event that the heat load on the evaporator is low over a long period of time, not all of the refrigerant will be evaporated. Otherwise, it might be possible that after a long period of low load there would be insufficient liquid present to evaporate to cause operation of the valve 50. By adjusting the operation of the valve 60 as described, a predetermined minimum amount of liquid refrigerant may be brought into the accumulator to compensate for that which is being removed during a low load period of substantial length.

In the operation of the system when the load on the evaporator is normal, the pressure drop across the restrictor 53 is substantial and sufficient for bellows 54 drop across the restrictor 53 is materially decreased, thereby permitting bellows 55 and spring 59 to close the orifice 58 of the valve 50. This permits the spring 65 in the valve 60to open the orifice 63 against the be]- lows 62, thereby permitting air in the line 38 from the valve 25 to be discharged through the opening 66 in the valve 60. This reduces the pressure on the diaphragm 40 of the feed valve 18, thereby permitting that valve to close and reducing, or stopping, the flow of liquid refrigerant into the accumulator.

It will be understood that for conditions intermediate the extremes, the feed to the accumulator will be correspondingly modulated.

After a period of low load during which the flow to the feed valve has been reduced, an increase on the load in the evaporator will produce a drop in the vapor line 12 across the restrictor 53 such as to again open the valve 50, thereby closing the valve 60 to permit the 1 valve 25 to control the feed valve 18 in accordance with the requirements of the system.

While the control elements described are components of an air system, it will be understood by those skilled in the art that other control systems may be em-. ployed including electrical and mechanical and combinations thereof, I 2

Further, while the illustrated system is of the type in which an accumulator supplies refrigerant to an evaporator and the head is measured in the supply line from the accumulator, the invention is adapted for other well-known system layouts in which the head of liquid refrigerant in the evaporator may be sensed.

-' What is claimed is:

1. In a refrigeration system having an evaporator, means for supplying refrigerant to the evaporator, meansfor receiving refrigerant from the evaporator,

Y and means responsive to the head of refrigerant in the evaporator for controlling the supply of refrigerant to the evaporator, the improvement comprising means sensing the flow rate of vaporized refrigerant from the evaporator and means interconnecting the flow rate sensing means with the means controlling the supply to the evaporator, said flow rate sensing means being operative to override the control means for controlling the supply to the evaporator, whereby during normal operation the flow to the evaporator is controlled by the supply control means, and whereby during periods of low load the flow to the evaporator is reduced or cut off in order to avoid the accumulation of an excess amount of refrigerant in the evaporator.

2. The invention of claim 1 in which the flow rate sensing means has means for maintaining a predetermined minimum head of refrigerant in the evaporator.

3. The invention of claim 1 in which a plurality of evaporators are connected in parallel in the system.

4. The invention of claim 1 in which the refrigeration system has compressor means which has a suction line, and in which an accumulator is connected to supply refrigerant to the evaporator and to receive refrigerant operating head of refrigerant in the evaporator, and

reducing the supply of refrigerant to the evaporator.

below that which is required to maintain such predeter-v mined head when the flow rate from the evaporator is reduced below a predetermined minimum.

6. The method of claim 5, and maintaining a predetermined head of refrigerant in the evaporator which is below said operating head when the flow rate from the evaporator is reduced below its said predetermined minimum. 

1. In a refrigeration system having an evaporator, means for supplying refrigerant to the evaporator, means for receiving refrigerant from the evaporator, and means responsive to the head of refrigerant in the evaporator for controlling the supply of refrigerant to the evaporator, the improvement comprising means sensing the flow rate of vaporized refrigerant from the evaporator and means interconnecting the flow rate sensing means with the means controlling the supply to the evaporator, said flow rate sensing means being operative to override the control means for controlling the supply to the evaporator, whereby during normal operation the flow to the evaporator is controlled by the supply control means, and whereby during periods of low load the flow to the evaporator is reduced or cut off in order to avoid the accumulation of an excess amount of refrigerant in the evaporator.
 2. The invention of claim 1 in which the flow rate sensing means has means for maintaining a predetermined minimum head of refrigerant in the evaporator.
 3. The invention of claim 1 in which a plurality of evaporators are connected in parallel in the system.
 4. The invention of claim 1 in which the refrigeration system has compressor means which has a suction line, and in which an accumulator is connected to supply refrigerant to the evaporator and to receive refrigerant from the evaporator, and is connected to the suction line, and the means sensing the flow rate of refrigerant from the evaporator is located in the suction line.
 5. The method of controlling the supply of refrigerant to the evaporator of a refrigeration system having a compressor, comprising continuously measuring the head of refrigerant in the evaporator, continuously sensing the flow rate of vaporized refrigerant from the evaporator, supplying refrigerant to the evaporator as required to maintain a predetermined operating head of refrigerant in the evaporator, and reducing the supply of refrigerant to the evaporator below that which is required to maintain such predetermined head when the flow rate from the evaporator is reduced below a predetermined minimum.
 6. The method of claim 5, and maintaining a predetermined head of refrigerant in the evaporator which is below said operating head when the flow rate from the evaporator is reduced below its said predetermined minimum. 